Translation enhancer element of the human amyloid precursor protein gene

ABSTRACT

The present invention is directed to a DNA element that enhances the translation of the human amyloid precursor protein (APP) gene. The enhancer may be incorporated into expression vectors to enhance recombinant protein production. In addition, the invention is directed to an assay that utilizes vectors containing the translation enhancer element for the purpose of identifying agents that modulate the expression of the human amyloid precursor protein. These agents will ultimately be used to suppress APP expression in patients with Alzheimer&#39;s disease.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No.09/188,118, filed Nov. 9, 1998 (now U.S. Pat. No. 6,310,197). U.S. Ser.No. 09/188,118 claims the benefit of U.S. provisional application No.60/065,175, filed on Nov. 12, 1997 (now abandoned).

STATEMENT OF THE GOVERNMENT SUPPORT

The work leading to this invention was supported by one or more grantsfrom the U.S. Government. The U.S. Government therefore has certainrights in the invention.

FIELD OF THE INVENTION

The present invention is directed to a nucleic acid element thatenhances the translation of the human amyloid precursor protein (APP)gene. This element may be ligated to other structural genes to enhancerecombinant protein production. In addition, it may be ligated toreporter gene sequences and used in assays for the purpose ofidentifying factors that alter the expression of APP. In addition thesequence can be used as a therapeutic target for down-regulating APPproduction.

BACKGROUND OF THE INVENTION

Alzheimer's disease develops as the result of a complex series of stepsthat ultimately lead to neuronal cell death and the loss of cognitivefunction. At present, two steps appear to be of particular importance.The first is a synthesis of the amyloid precursor protein (APP) and itsprocessing into the Aβ peptides, which then polymerize and deposit asthe amyloid filaments that are the hallmark of Alzheimer's disease(Selkoe, J. Biol. Chem. 271:18295 (1996); Scheuner; et al., Nature Med.2:864 (1996); Goate, et al., Nature 349:704 (1991)). Coupled to thisprocess is a special form of inflammation and acute phase response inthe brain that leads to an increase in the production ofamyloid-associated proteins, α₁-antichymotrypsin (ACT) and complementactivation (Abraham, et al., Cell 52:487 (1989)). In vitro studies haveshown that ACT and another amyloid-associated protein, apolipoprotein-E(ApoE), regulate the polymerization of Aβ peptides into amyloidfilaments (Yee, et al., Nature 372:92 (1994)). The ApoE 4 and, possibly,the ACT-A alleles are inherited risk factors for Alzheimer's disease(Corder, et al., Science 261:921 (1992)).

Several facts suggest a direct connection between increased APP levelsand the development of Alzheimer's disease and further suggest that suchan increase may be linked to inflammatory mechanisms:

-   -   a) Down syndrome brains in trisomy-16 mice show increased APP        protein levels beyond the 0.5-fold increase that would be        expected by gene dosage (Neve, et al., Mol. Brain Res. 39:185        (1996)).    -   b) Over-expression of APP protein in transgenic mice is        necessary, even in the presence of FAD mutations, for sufficient        Aβ peptide production to lead to the development of amyloid        filament deposits and an Alzheimer's-like pathology (Quon, et        al., Nature 352:239 (1991)). Furthermore, APP protein synthesis        correlates with Aβ peptide production both in vitro and in vivo        (Ho, et al., J. Biol. Chem. 271:30929 (1996)).    -   c) Traumatic brain injury, a known risk factor for Alzheimer's        disease, increases IL-1 as well as APP-immunoreactivity in rat        brain (Nieto-Sampedro, et al., J. Neurosci. Res. 17:214 (1987)).    -   d) IL-1 injected into the rat cerebral cortex increases the        steady-state levels of APP protein at the site of the lesion        (Sheng, et al., Neurobiol. Aging 17:761 (1996)) and primary        astrocytes have been shown to be a source of secreted Aβ        peptides (Busciglio, et al., Proc. Natl. Acad. Sci. U.S.A.        90:2092 (1993)).

The identification of the mechanisms by which inflammation leads to theoverproduction of APP in brain cells may lead to new therapies forcontrolling Alzheimer's disease. Beyond this, the discovery of newmethods and elements for regulating gene expression will provide newopportunities for controlling the production of recombinant genes bothin vitro and in vivo.

SUMMARY OF THE INVENTION

The present invention is based upon the discovery of a distinct DNAelement that increases the rate at which the mRNA transcribed from theamyloid precursor protein (APP) gene is translated. This element may becombined with other genes to increase recombinant protein productionwithout increasing transcriptional activity.

In its first aspect, the invention is directed to a substantially pureDNA molecule comprising the translation enhancer element of APP operablylinked to a non-homologous gene, i.e., a gene other than that encodinghuman APP. The translation enhancer element consists essentially of thenucleotide sequence of SEQ ID NO:1 and, in a preferred embodiment, thenon-homologous gene is located at a site between 10 and 100 nucleotides3′ to the last 3′ nucleotide in the enhancer.

In another aspect, the present invention is directed to a vector forrecombinantly expressing a peptide or protein in a eukaryotic cell. Thevector contains a promoter active in the cell; a translation enhancerelement having a sequence consisting essentially of that of SEQ ID NO:1lying 3′ to the promoter; and 5′ to a DNA sequence encoding the peptideor protein for recombinant production. The sequence encoding the peptideor protein should be located 3′ to the enhancer element; be operablylinked to the promoter; and be non-homologous with respect to thetranslation enhancer element. In a preferred embodiment, the geneundergoing recombinant expression is located at a site between 10 and100 nucleotides 3′ to the last 3′ nucleotide in the enhancer. Thesevectors may be used to transform a variety of host cells, preferablyeukaryotic host cells, using standard techniques for transformation.Cells transformed in this manner are also within the scope of thepresent invention.

The invention is also directed to a method for recombinantly producingprotein in which host cells transformed with the vector discussed aboveare grown, in vitro or in vivo, and recombinant protein is then purifiedeither from the host cells or from the growth medium surrounding thecells. Purification may be accomplished by standard biochemicaltechniques including precipitations, chromatography on various matrices,electrophoretic techniques, affinity chromatography, etc. Optionally,the method may include exposing host cells to an inducer, e.g. acytokine such as interleukin-1α and interleukin-1β, that increases theactivity of the translation enhancer element. An optimal concentrationof inducer can be determined by titrating it into the system andmeasuring the amount of recombinant protein produced at eachconcentration. In addition to being directed to such methods, theinvention includes the recombinant peptides or proteins that areproduced by these methods.

In another aspect, the present invention is directed to a method forassaying test compounds for their ability to alter the expression ofhuman APP. This may be accomplished by preparing a vector containing apromoter, the translation enhancer element, and a non-homologous geneoperably linked to the element. Preferably, the non-homologous gene willproduce a product that can be quantitated with relative ease, e.g., thechloramphenicol acetyltransferase gene may be used for this purpose.Gene expression is then measured in the presence and absence of the testcompound in order to determine whether there is either an enhancement orinhibition of expression. Assays may be carried out either using invitro systems or after transforming host cells with the vector. Becauseover-expression of APP has been associated with Alzheimer's disease,agents that inhibit the activity of the translation enhancer element areof particular interest. Thus, the present invention includes methods inwhich the test compounds used are antisense agents specifically directedto the translation enhancer element. These antisense compounds should benucleic acids complementary to a region of SEQ ID NO:1 that is at leastten bases in length. Agents of this type may undergo a variety ofmodifications to increase their effectiveness. Other test compounds thatcan be used in the assays include RNA targeting compounds that alter theenhancer function of the sequence. Such compounds may act by recognizingportions of the secondary structure assumed by different RNAs. Inaddition pharmacological reagents and inhibitory receptor-mediatedligands may be tested.

Definitions

The invention description provided herein uses a number of terms thatrefer to recombinant DNA technology. In order to provide a clear andconsistent understanding of the specification and claims, including thescope to be given such terms, the following definitions are provided.

Substantially pure: As used herein, the term “substantially pure” refersto a biological component, protein or nucleic acid, that has beenseparated from other accompanying biological components so that,typically, it comprises at least 85 percent of a sample, with greaterpercentages being preferred. Many means are available for assessing thepurity of nucleic acids and proteins within a sample, including analysisby polyacrylamide gel electrophoresis, chromatography and analyticalcentrifugation.

Operably linked: The term “operably linked” refers to genetic elementsthat are joined in a manner that enables them to carry out their normalfunctions. For example, a gene is operably linked to a promoter when itstranscription is under the control of the promoter and the transcriptproduced is correctly translated into the protein normally encoded bythe gene.

Consists essentially of: The term “consists essentially of,” or“consisting essentially of,” is used in conjunction with the sequence ofthe translation enhancer element. It indicates that the translationenhancer encompasses sequences exactly the same as that shown in SEQ IDNO:1, as well as DNA elements with differences that are not substantial,as evidenced by their retaining the basic, qualitative functionalproperties of the element. In particular, it is anticipated that minorsubstitutions, additions or deletions of nucleotides may take placewithin the sequence at positions that do not affect its ability toenhance the translation of an operably linked gene.

Non-homologous: The term “non-homologous” is used herein to indicatethat the APP translation enhancer element is joined to a gene other thanthe one it would normally be joined to in nature, i.e., the translationenhancer element is joined to something other than the human APP gene.

Promoter: A promoter is the DNA sequence at which transcription isinitiated. If the promoter is of the inducible type, then its activityincreases in response to an inducing agent.

Complementary Nucleotide Sequence: The term “complementary nucleotidesequence,” refers to a sequence that would arise by normal base pairing.For example, the nucleotide sequence 5′-AGA-3′ would have thecomplementary sequence 5′-TCT-3′.

Expression: Expression is the process by which a polypeptide is producedfrom DNA. The process involves the transcription of the gene into mRNAand the subsequent translation of the mRNA into a polypeptide.

Host: Any prokaryotic or eukaryotic cell that is the recipient of areplicable expression vector or cloning vector is the “host” for thatvector. The term encompasses prokaryotic or eukaryotic cells that havebeen engineered to incorporate a desired gene on its chromosome or inits genome. Examples of cells that can serve as hosts are well known inthe art, as are techniques for cellular transformation (see e.g.,Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd ed. ColdSpring Harbor (1989)).

Cloning vector: A cloning vector is a DNA sequence (typically a plasmidor phage) which is able to replicate autonomously in a host cell, andwhich is characterized by one or a small number of restrictionendonuclease recognition sites. A foreign DNA fragment may be splicedinto the vector at these sites in order to bring about the replicationand cloning of the fragment. The vector may contain one or more markerssuitable for use in the identification of transformed cells. Forexample, markers may provide tetracycline or ampicillin resistance.

Expression vector: An expression vector is similar to a cloning vectorbut is capable of inducing the expression of the DNA that has beencloned into it, after transformation into a host. The cloned DNA isusually placed under the control of (i.e., operably linked to) certainregulatory sequences such as promoters or enhancers. Promoter sequencesmay be constitutive, inducible or repressible.

Gene. As used herein, the term “gene” refers to a nucleic acid sequencecontaining a template for a nucleic acid polymerase, in eukaryotes, RNApolymerase II. Genes are transcribed into mRNAs that are then translatedinto protein.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a translation enhancer element thatwas first identified in the 5′ untranslated region (5′ UTR) of the humanamyloid precursor protein gene. The element is defined by its structureas shown in SEQ ID NO:1. However, it will be understood that theinvention encompasses not only sequences identical to that shown, butalso sequences that are essentially the same as evidenced by theirretaining the basic functional characteristic of enhancing thetranslation of an operably linked structural gene. In addition, thepresent invention encompasses methods of recombinantly producing proteinwhich utilize this element and methods for assaying compounds for theirability to inhibit APP expression.

I. APP Translation Enhancer Element

The APP translation enhancer element is 90 nucleotides in length and maybe obtained by a wide variety of methods. One method that has proven tobe effective is to isolate the element from the 5′ untranslated regionof a human APP clone, obtained by screening the cDNA library of a celltype known to produce large amounts of APP. For example, the procedureof Kang, et al. (Nature 325:733 (1987)) can be used to clone the APPcDNA by expression screening a library of fetal brain cDNAs. Other cellsthat may be utilized include human astrocytoma cells and humanastrocytes.

Once the complete APP cDNA has been isolated, the translation enhancerelement may be obtained by digesting clones with appropriate restrictionenzymes and subcloning fragments containing the element. In the case ofthe library discussed above, the CD plasmids containing the cloned APPcDNA can be digested with a combination of SmaI and HindIII to obtain a3 kB fragment. This may then be incorporated into either a cloningvector or an expression vector. For example, the SmaI/HindIII digestionproduct may be inserted into compatible StuI/HindIII sites in the 5′ UTRof pSV₂CAT. The 3 kB APP gene body may then be cut out of this clone(designated pSV₂(APP-1)CAT) by digestion with NruI and HindIII to leavebehind the APP gene 5′ UTR fused to the CAT reporter gene.

The plasmid produced in this manner may be transfected into host cellsusing standard techniques (i.e., calcium phosphate precipitation,liposome transfer, electroporation, etc.) and the host cells grown toproduce large amounts of plasmid. Alternatively, host cells may be usedin the assays described in section III below. When cells, e.g.astrocytoma cells, are transfected with pSV₂(APP)CAT, they produce achimeric transcript in which 90 nucleotides of the APP gene 5′ UTR arepart of a 117 nucleotide 5′ leader sequence.

Although the above procedure is suitable for obtaining the human APPtranslation enhancer, many alternative techniques have been describedfor isolating genetic elements and it is expected that these can beadapted to the isolation of the APP translation enhancer with relativelylittle effort (see e.g., Sambrook, et al., Molecular Cloning: ALaboratory Manual, 2nd ed., Cold Spring Harbor Press (1989)). Thus, theenhancer element my be chemically synthesized or cDNA libraries may bescreened using labeled PCR-generated probes corresponding to regions ofthe known APP gene sequence. In general, such probes should be at least14 nucleotides long and should not be selected from a region known to beconserved among proteins. In one especially preferred alternative, theAPP gene sequence may be used to construct PCR primers for the purposeof amplifying the enhancer element.

II. Method of Recombinantly Producing Protein

One of the main uses for the human APP translation enhancer element isin the recombinant production of protein. To make an appropriateexpression vector, the techniques discussed above can be used to obtainthe enhancer element which should then be positioned downstream from thestart site of transcription and upstream from the structural genesegment undergoing expression. The exact position relative to thepromoter and gene cap site is not critical to the invention but,preferably, the cap site will be between 10 and 100 nucleotides 3′ tothe last 3′ nucleotide in the enhancer. In all cases, the enhancershould be between the AUG codon and the transcription promoter. Otherelements present will vary depending upon host cell type, but willgenerally include sequences involved with the initiation oftranscription and translation and sequences signaling the termination oftranscription Transcriptional enhancer sequences may also be present.Examples of eukaryotic promoters that may be used include the promoterof the mouse metallothionein I gene (Haymer, et al., J. Mol. Appl. Gen.1:273 (1982)); and TK promoter of Herpes virus (McKnight, Cell31:355-365 (1982)); the SV40 early promoter (Benoist, et al., Nature290:304 (1981)), etc.

It is widely known that translation of eukaryotic mRNA is initiated atthe codon which encodes the first methionine of a gene. For this reason,the linkage between a promoter and the DNA structural sequence shouldnot contain any intervening codons for methionine. The presence of suchcodons results either in the formation of a fusion protein (if the AUGcodon is in the same reading frame as the structural sequence) or aframe-shift mutation (if the AUG codon is not in the same readingframe). The insertion of the enhancer should not, in itself, generateany misplaced start codons

A large number of plasmids suitable for use in eukaryotes have beendescribed (Botstein, et al., Miami Winter Symp. 19:265 (1982); Broach,Cell 28:203 (1982); Bollon, et al., J. Cin. Hematol. Oncol. 10:39(1980); Maniatis, in Cell Biology: A Comprehensive Treatise, vol. 3,Academic Press, M.Y. pp. 563-608 (1980)). In addition, the translationenhancer element may be incorporated into DNA constructs designed forhomologous recombination (see Capecchi, TIG 5:70 (1989); Mansour, etal., Nature 336:348 (1988); Thomas, et al., Cell 51:503 (1987); andDoetschman, et al., Nature 330:576 (1987)).

Once the vector or DNA sequence has been prepared, it may be introducedinto an appropriate host cell by any suitable means of transfection(e.g., calcium phosphate and lipofectin precipitation). Large amounts ofrecipient cells may then be grown in a medium which selects forvector-containing cells. If desired, an inducer may be introduced intothe growth medium for the purpose of increasing the activity of thetranslational enhancer element. Inducers that have been found to beeffective in this regard are interleukin-1α and interleukin-1β but it ispossible that other cytokines may be used as well.

The expressed recombinant protein may be purified in accordance withconventional methods such as extraction, precipitation, chromatography,affinity chromatography, electrophoresis and the like. The exactprocedure used will depend upon both the specific protein produced andthe specific expression system utilized.

III. Assay for Compounds Modulating APP Expression

Overproduction of the APP protein has been closely associated with thedevelopment of Alzheimer's disease. Therefore, assays for theidentification of compounds that either inhibit or enhance expressionare of considerable interest. Compounds that inhibit expression havepotential use as therapeutic agents whereas compounds enhancingexpression would have use in scientific studies examining thepathogenesis of Alzheimer's disease.

Assays may be performed using an expression vector in which the APPtranslation enhancer is located upstream from a reporter gene. Forexample, the pSV₂(APP)CAT plasmids described in section I may beutilized. These plasmids should be transfected into appropriate hostcells, e.g., astrocytes or astrocytoma cells, which are then dividedequally into sample wells and exposed to test compounds. The effect ofthe test compounds on reporter gene expression can then be determined bycomparing the expression seen in the presence of test compound with thattaking place in its absence.

In order to confirm that compounds are acting at the level oftranslation, the mRNA content of exposed and unexposed cells may becompared (see Examples section for a description of one procedure thatcan be used for this purpose). If desired, assays may be carried out inthe presence of cytokines such as interleukin-1α or interleukin-1β todetermine whether test compounds alter the enhancement of translationalactivity typically seen with these compounds.

One group of test compounds that are of particular interest areoligonucleotides complementary to segments of the translation enhancersequence. These oligonucleotides should be complementary to at least 10bases within the enhancer element and, preferably, 15 bases or more.Oligonucleotides which are found to alter translational activity may bederivatized or conjugated in order to increase their effectiveness. Forexample, nucleoside phosphorothioates may be substituted for theirnatural counterparts (see Cohen, Oligodeoxynucleotides, Antisense,Inhibitors of Gene Expression, CRC Press (1989)). The oligonucleotidesmay also be designed for delivery in vivo for the purpose of inhibitingAPP expression. When this is done, it is preferred that theoligonucleotide be administered in a form that enhances uptake by cells.For example, the oligonucleotide may be delivered by means of aliposome, retrovirus, or conjugated to a peptide that is ingested bycells (see, e.g., U.S. Pat. Nos. 4,897,355 and 4,394,448). Other methodsfor enhancing the efficiency of oligonucleotide delivery are well-knownin the art and are also compatible with the present invention.

RNA targeting compounds, pharmacological reagents and inhibitoryreceptor-mediated ligands may also be tested in the assays. The mostpreferred of these are RNA targeting compounds that act by recognizingthe secondary structure that results from the folding of RNA.

EXAMPLES

I. Methods

Preparation of Primary Human Astrocytes

Primary human astrocytes were prepared by trypsinization of human fetalbrain tissue by a previously described method (Das, et al., Neuron 14;447 (1995)). Cells were seeded onto poly-L-lysine (10 μg/ml) coatedplates and were grown to 70% confluence in DMEM medium, (low glucose)supplemented with 10% fetal bovine serum. In order to eliminatemicroglia, the cultures were treated with 5 mM H-Leu-O-methylester(Guilian, J. Neurosci. Res. 18:155 (1987)). For immunofluorescence,cells were replated at a density of 100,000 cells per 60-mm petri dishon poly-L-lysine coated glass coverslips and grown for 24 hours in 0.5ml of medium to allow the cells to attach. The presence of glialfibrillary acidic protein (GFAP) was detected by immunofluorescentstaining of formaldehyde fixed cells using a 1:10 dilution of rabbitIgG₁ antibody specific for human GFAP. Primary GFAP-specific antibodywas added to the coverslips for 30 minutes at room temperature inphosphate buffered saline and 10% goat serum. The secondary antibodiesused were goat anti-rabbit IgG conjugated to fluorescein isothiocyanateat a dilution of 1:100 (Boehringer). Fluorescence microscopy scored thecells used in the metabolic experiments as 95% pure astrocytes.Antibodies to neuronal specific microtubule associated protein (MAP-2)and beta-3-tubulin failed to label the astrocyte cultures. The U373MGastrocytoma cell line was cultured on 100 mm uncoated dishes to 60-80%confluence in DMEM medium supplemented with 10% fetal bovine serum.

Determination of APP Protein Synthesis

Intracellular APP protein synthesis was determined in primary astrocytesafter plating cells in equal numbers into 8 microtiter wells prior toeach treatment (1×10⁵ cells per well in 96 well dishes). Astrocytes in 5rows of 12 wells were stimulated for 16 hours with: (1) 0.5 mg/mlrecombinant IL-1α; (2) 0.5 mg/ml recombinant IL-1β; (3) iron (deliveredas ferrotransferrin, 10 mM Fe₂Tf, and chelated with 10 mM-100 mMdesferrioxamine (Van Nostrand, et al., Proc. Natl. Acad. Sci. U.S.A. 88:10302 (1991)); (4) desferrioxamine; or (5) left untreated as controls(1×10⁵ cells). Cells from random wells were counted in order to ensure aconsistent presence of 1×10⁵ cells per well at the beginning of eachexperiment. Astrocytes were preincubated for 15 minutes inmethionine-free medium and pulse-labeled with 300 μCi/ml[³⁵S]-methionine for 30 minutes in methionine-free medium (RPMI 1640;GIBCO). Each microtiter plate was washed twice in cold phosphatebuffered saline (PBS) at 4° C. before lysis of astrocytes with 25 mlSTEN buffer and a sterile glass rod. STEN buffer is 0.2% NP-40, 2 mMEDTA, 50 mM Tris, pH 7.6. The addition of 20 mM PMSF, 5 mg/ml leupeptinto the lysis buffer prevented proteolysis. The buffers from each wellwere pooled into a total volume of 300 microliters. One half of eachpooled lysate was immunoprecipitated with antiserum raised against thecarboxyl terminus of APP (1:500 dilution of C-8 antibody raised againstamino acid residues 676-695 of APP-695). The remaining portion of eachlysate was immunoprecipitated with human ferritin antiserum (1:500dilution, Boehringer, Indianapolis, Ind.).

Secretion and labeling of APP(s) (protease-nexin-2) from primaryastrocytes by IL-1 was measured in a separate set of experiments. Mediumfrom 100 mm dishes (10 ml) of astrocyte cultures was collected after a2-hour pulse labeling with 200 μCi/ml [³⁵S]-methionine. APP in 5 ml ofprecleared medium was immunoprecipitated using a 1:1500 dilution ofrabbit polyclonal serum specific for amino acids 595-611 of APP (R1736,D. Selko). Apolipoprotein E was immunoprecipitated from 5 ml of culturesupernatant using a 1:200 dilution of a polyclonal antiserum (Chemicon).

Preparation of Astrocytoma Cells

Astrocytoma cells (60% confluent) were stimulated with IL-1 at the sameconcentrations as used for primary astrocytes. After stimulation, equalnumbers of cells were labeled with 100 μCi/ml [³⁵S]-methionine in DMEMmedium lacking methionine, washed twice with PBS, and the cell pelletslysed in 200 μl cold STEN buffer containing 20 mM PMSF. APP wasimmunoprecipitated by adding 2 μl of anti-APP antibody (C-8).

Immunoprecipitations

In all labeling experiments, immunoprecipitated protein was collected bythe binding of antibody-labeled antigen complexes to Protein ASepharose™ beads. Immunoprecipitated samples were applied to 10-20%tris-tricine gels (Novex) and the samples were electrophoresed intris-tricine buffer according to the manufacturer's instructions. Thegels were fixed with 25% methanol, 7% (v/v) methanol for 1 hour, treatedwith fluorographic reagent (Amplify, Amersham) for 30 minutes, dried,and exposed to X-omat Kodak film overnight at −80° C.

Northern Hybridizations

Total RNA (10 μg) was extracted from primary astrocytes and astrocytomacells with a RNA-STAT kit (Tel-Test). RNA samples were denatured in 50%formamide/2.2 M formaldehyde/20 mM MOPS/5 mM sodium acetate/0.5 mM EDTA,pH 7.4, at 60° C. for 10 minutes, electrophoresed on 1.0%agarose-formaldehyde gels, blotted onto Hybond-N filters and immobilizedby UV crosslinking and heating filters to 80° C. Filters wereprehybridized for 3 hours and hybridized overnight in a solutionconsisting of 50% formamide, 50 mg/ml denatured salmon sperm DNA, 5×SSC,0.1% sodium dodecyl sulfate and 5×Denhardt's solution. Followinghybridization, filters were washed twice for a total of 1 hour in2×SSC/0.2% sodium dodecyl sulfate at room temperature and twice each fora total of 1 hour in 0.5×SSC/0.1% sodium dodecyl sulfate at 55° C. Equalloading was verified by ethidium bromide staining, non-specifichybridization of the ACT probe to the 28S rRNA, and by standardizedhybridization to the GAPDH cDNA probe as an internal standard. The APPcDNA probe corresponded to the unique internal 1 kb fragment gelpurified from the 3 kb APP cDNA (Kang et al. Nature 325: 733 (1987)),the ACT cRNA probe to a PstI/SacI fragment (536-943) in the human ACTcDNA (Chandra, et al., Biochemistry 22: 5055 (1983)) and the GAPDH probeto the human GAPDH gene (Tokunaga, et al., Cancer Res. 47: 5616 (1987)).

Construction of pSV-₂(APP)CAT

The pSV₂(APP)CAT construct contains the APP gene 5′ UTR in between theSmaI and the NruI sites (+52 nt and +142 nt from the 5′ cap siterespectively). pSV₂(APP)CAT was prepared by two steps of subcloning. (1)A 3 kb SmaI-HindIII fragment containing the APP gene, including thecoding region and a segment of the 3′ UTR was subcloned into compatibleStuI-HindIII sites unique to the 5′ UTR of the CAT gene in the pSV₂CATexpression vector. (2) The APP gene fragment in between the NruI andHindIII sites was removed from the construct. The restriction sites werethen blunt-ended and religated. In the pSV₂(APP)CAT transfectants, 90 ntof the APP gene 5′ UTR were expressed as part of a chimeric 1171 nt 5′leader in the 1.5 kb APP/CAT transcript. The pSV₂CAT construct containsthe 5′ end of the CAT gene inserted into the unique polylinker site inpBluescript. A 250 nt HindIII/EcoRI fragment from the CAT gene inpSV₂CAT was subcloned into the pBS vector (Stratagene). The CAT genefragment codes for 36 nt of the CAT gene 5′ UTR and 218 bp of the 5′ endof the coding sequence of the CAT gene.

Transfections

Astrocytoma and neuroblastoma cells were transfected with pSV₂CAT orpSV₂(APP)CAT by lipofection. Briefly, lipofectamine reagent (Boehringer)was added to DMEM (without serum) and allowed to sit 30 minutes at roomtemperature. Plasmids (10 μg) in an equal volume of DMEM were thenadded, and, after sitting for 45 minutes at room temperature, thelipid/DNA solution was added to 60% confluent cells on 100×20 mm cellculture plates. After 4 hours, this solution was removed, and the cellswere washed twice in DMEM (without serum). Fresh DMEM medium (containing10% fetal calf serum) was added. At this time, treatments wereadministered which included: (1) 0.5 ng/ml IL-1α; (2) 0.5 ng/ml IL-1β;(3) 5 μM Fe₂TF (holo-transferrin); and (4) unstimulated controls. After20 hours, cells were harvested in phosphate buffered saline (PBS) andimmediately assayed for CAT activity or mRNA levels (RNase protection).

CAT Activity

After harvesting, cells were resuspended in 100 μl 0.25 M Tris, pH 7.8,and subjected to three cycles of freezing (liquid nitrogen) and drying(37° C.) to lyse cells. Lysates were collected after centrifugation at10,000 rpm for 5 minutes. Protein concentration was determined by aBiorad assay, and exactly 20 μg of lysate was added to a CAT reactionmix containing 50 μl 1M Tris, pH 7.8, 20 μl acetylcoenzyme-A (3.5 mg/ml)and 5 ml ¹⁴C-labeled chloramphenicol (25 mCi/ml). After 1 hour at 37°C., reaction products were extracted with 1 ml ethyl acetate and thesamples were resolved by thin layer chromatography as describedpreviously (Rogers, Blood 87:2525 (1996)). For quantitative analysis,the areas on the TLC plates which aligned with dots on the film wereexcised and the radioactivity was counted in 5 ml of scintillationcocktail (Econofluor) using a scintillation counter (Hewlett Packard).In some experiments, CAT activity was measured by counting the amount ofCAT reaction product diffusing into liquid scintillation fluid asdescribed previously (Rogers, et al., Nucl. Acid Res. 22:2678 (1994)).Each lysate was incubated at 37° C. with [³H]-acetylcoenzyme-A (0.1 mCi)and 2 mM chloramphenicol (CAP) in 200 ml of 100 mm TrisCl (pH 7.8). Thisaqueous reaction mix had been overlaid by 5 mls of liquid scintillant(Econofluor, NEN).

RNase Protection Experiments

Cells were lysed in 2 ml of a buffer containing 4 M guanidiniumthiocyanate, 25 mM NaOac, pH 6.0, 100 mm β-mercaptoethanol. RNA wasprepared after shearing the DNA and centrifuging the lysate at 31K, 23°C. for 12 hours through 5 ml of 5.7 M CsCl₂ cushion. This procedureensured that CAT mRNAs and endogenous ferritin mRNAs were purified as apellet without contamination from plasmid DNA containing transfected CATgene sequences. The RNA pellet was resuspended in TES buffer (10 mMTris, pH 7.6, 1 mM EDTA, 0.5% SDS) and was extracted with an equalvolume of phenol/chloroform, ethanol-precipitated and resuspended in TESbuffer.

Steady-state levels of transfected CAT mRNA from astrocytoma andneuroblastoma pSV₂(APP)(CAT) transfectants was characterized by RNaseprotection. A 261 nucleotide ³²P-labeled cRNA was transcribed from aHindIII digested DNA template isolated from the CAT gene in the pSV₂CATsubclone (Rogers, et al., Nucl. Acid Res. 22: 2678 (1994); Campbell, etal., Biochem. Biophys. Res. Commun. 160: 453 (1989); Fahmy, et al.,Biochem. J. 296; 175 (1993)). Labeled cRNA, antisense to the CAT gene,was hybridized to 20 μg of mRNA purified from pSV₂(APP)(CAT)transfectants of astrocytoma cells. Hybridization was for 24 hours at45° C. in a buffer containing 80% formamide, 40 mM Pipes (pH 6.7), 0.4 MNaCl, 1 mM EDTA. Digestion with RNase A (40 mg/ml) and RNase T1 (2mg/ml) removed unhybridized cRNAs. Protected cRNAs were separated byelectrophoresis through a 6% polyacrylamide/urea sequencing gel. Kinaselabeled HaeIII-digested X-174 DNA fragments were used as DNA markers forautoradiography and quantitation of CAT mRNA levels.

II. Results and Discussion

APP protein synthesis was measured in primary human fetal astrocytesfollowing treatment with 0.5 ng/ml of IL-1α or IL-1β for 16 hours.Experiments were performed in which a 30 minute metabolic labeling with[³⁵S]-methionine was followed by APP immunoprecipitation and gelelectrophoresis. These indicated that there was a 4-fold increase in thesynthesis of intracellular APP in response to IL-1α and a 3-foldincrease in response to IL-1β. There was also a 4-fold and a significant25% up regulation in H-ferritin and L-ferritin subunit synthesisrespectively. These experiments show that the APP gene is regulated byIL-1 at the translational level.

IL-1 also induced secretion of APP (APPs; protease-nexin II) fromprimary human astrocytes. Medium was collected after a 2 hour pulselabeling with [³⁵S]-methionine, immunoprecipitated using a N-terminaldirected antibody raised against amino acids 595-611 of APP (Sekoe, etal., Proc. Nat'l Acad. Sci. USA 85:7341 (1988)) and analyzed by gelelectrophoresis and autoradiography. Quantitation of theimmunoprecipitate showed a 1.8-fold enhanced secretion of APP(s) intothe medium induced by IL-1β and a smaller, 50%, increased accumulationof APP(s) in response to IL-1α. Thus, the levels of both cell-associatedand secreted APP were increased by exposure of primary astrocytes toIL-1. The synthesis of the 36 kDa Apo-E protein was measured as aninternal standard of IL-1 stimulation and metabolic labeling with[³⁵S]-methionine. In both cases no increase in ApoE protein synthesiswas observed.

In contract to the IL-1α induced APP protein synthesis in primaryastrocytes, there was no observable increase in APP-mRNA levels inmeasurements by Northern blot analysis indicating that the effect ofIL-1 on APP synthesis was at the level of translation. As a control,IL-1α was found to stimulate a pronounced increase in the steady statemRNA levels of alpha-1 antichymotrypsin (ACT) in pure human astrocytes,as has previously been reported (Das, et al., Neuron 14:447 (1995)).Densitometry showed that IL-1α exposure left APP-mRNA levels unchangedwhile inducing a greater than 10-fold increase in ACT-mRNA levels.Primary astrocytoma cells were grown from a separate fetal brain cortexsample source and, in this confirmatory experiment, GAPDH mRNA levelswere used to standardize for loading differences between lanes.

In order to extend the findings with primary human fetal brainastrocytes, the effect of both IL-1α and IL-1β in human astrocytoma(U373MG) cells was tested. Cells were grown to 80% confluence, and equalnumbers of cells were stimulated with IL-1α (0.5 ng/ml), IL-1β (0.5ng/ml) or left unstimulated for 16 hrs. IL-1α as well as IL-1β increasedthe rate of APP synthesis (maximal 2.8 fold and 4.3 fold respectively)and enhanced the rate of ferritin-H subunit synthesis by 5-fold andL-ferritin by 2 fold in astrocytoma cells. While IL-1α is a more activeinducer than IL-1β of APP in primary astrocytes the reverse is true inastrocytoma cells.

As was found for primary astrocytes, IL-1α stimulation of APP geneexpression in astrocytoma cells was at the level of translation. In fourseparate Northern blot experiments, IL-1β stimulated only an average 30%increase in the steady-state levels of APP-mRNAs as standardized toGAPDH-mRNA expression, whereas the cytokine increased APP proteinsynthesis by 4.3-fold during the same experiment. At the same time,IL-1α and IL-1β each stimulated a large transcriptional increase of ACTmRNA observed in primary human astrocytes. In Northern blots measuringthe steady state levels of APP-mRNA and ACT mRNA after 16 hours of IL-1stimulation, it was found that ACT-mRNA levels in unstimulated cellswere undetectable. Low ACT-mRNA expression in unstimulated cells wasdetected after 48 hours exposure of the blots and allowed forquantitation of a 6-fold induction in steady-state levels of ACT-mRNA.

The translational enhancer region of the L-ferritin gene 5′UTR showssignificant sequence alignment with the APP-mRNA 5′ leader. A 51%sequence alignment between the L-ferritin and APP-mRNA 5′ UTRs wasconfirmed by computer searching of the APP mRNA (Gap program, GDGDefssoftware from Univ. of Wisconsin, Madison, Wis.). For this reason, theAPP mRNA 5′ UTR was considered to be an excellent candidate to carrysequences capable of IL-1-dependent translation enhancement. Previously,the 5′ untranslated regions of the L-ferritin and H-ferritin genes (+74to +142 from the L gene cap site and +139 to +199 from the H-gene capsite) had been shown to confer both baseline and IL-1β dependenttranslation to a chloramphenicol acetyltransferase (CAT) reporter genetransfected in human hepatoma cells (Rogers, et al., Nucl. Acid Res.22:2678 (1994); Campbell, et al., Biochem. Biophys, Res. Commun. 160:453(1989); Fahmy, et al., Biochem. J. 296:175 (1993); Rogers, Blood 87:2525(1996)). Therefore a pSV-₂(APP)CAT reporter construct was prepared inwhich sequences from positions at +55 to +144 nt of the 146 nt APP mRNA5′ UTR were inserted immediately upstream of a hybrid CAT reporter mRNAstart codon.

The APP mRNA 5′ UTR conferred IL-1 dependent translational enhancementto CAT reporter mRNAs in pSV₂(APP)CAT-transfected astrocytoma cells. TheAPP mRNA 5′ UTR mediated a maximal 3-fold and 4-fold increase in CATactivity following stimulation with IL-1α and IL-1β respectively. IL-1βstimulation of astrocytoma cells transfected with the parental vectorpSV₂CAT had no effect on CAT activity. These results confirmed that theparental pSV₂CAT vector is unresponsive to IL-1 and that the APP-mRNA 5′UTR is important for mediating translational regulation by IL-1 inastrocytoma cells. In a parallel transfection, the IL-1β stimulus causeda 3-fold enhancement of CAT activity in pSV₂(APP)CAT transfectants.

The increased CAT activity in the pSV₂(APP)CAT-transfected astrocytomacells following stimulation with either IL-1α or IL-1β was notaccompanied by any major changes in APP/CAT mRNA transcription. RNAseprotection analysis demonstrated that the steady-state APP/CAT mRNAlevel was modestly increased (30%) in pSV₂(APP)CAT transfectedastrocytoma cells relative to unstimulated cells after 16 hours of IL-1βstimulation. Densitometric evaluation of two separate transfectionsshowed that 16 hours of IL-1α stimulation decreased the steady-statelevel of CAT mRNA by 50% in pSV₂(APP)CAT transfectants, while matchingexperiments evidenced an average 2.3-fold (maximal 4-fold) increase inCAT activity. In addition to mediating IL-1 dependent translation, theAPP 5′UTR conferred a consistent 6-fold increase in basal CAT activityin pSV₂(APP)CAT transfectants compared to the parental astrocytoma cellstransfected with pSV₂CAT. In this experiment transfections werestandardized with the RSV₂GAL plasmid, and the amount of CAT geneexpression was calculated after transfection efficiencies were takeninto account. It was concluded that the APP mRNA 5′ UTR acute boxsequences enhance IL-1-stimulated translation of APP and also increasebaseline activity. This explains the consistent differences in baselineCAT gene expression resulting from pSV₂(APP)CAT transfection compared totransfection with pSV₂CAT. A similar increase in baseline CAT geneexpression has been found after transfection of the light and heavyferritin mRNA acute boxes into hepatoma cells (Rogers, Blood 87:2525(1996)).

The data show that IL-1 induces APP protein synthesis by a mechanism ofenhanced message translation in two different cellular systems, both ofastrocytic origin. This is the second example of translationalregulation by IL-1. This cytokine was previously shown to regulatehepatic ferritin translation which may account for part of the anemia ofchronic diseases. The most straightforward interpretation of the resultsis that the primary inflammatory cytokine, IL-1 elevates APP-mRNAtranslation through the action of an IL-1 responsive stem-loop upstreamof the APP gene coding region.

One previous report directly indicates that APP gene expression may becontrolled at the level of message translation. APP-mRNA was shown to beexpressed as two major forms of mRNA in the human brain resulting frompolyadenylation of two poly(A) selection sites (PA-1 and PA-2). Thelonger APP mRNA (3.3 kB) was found to be translated 3-fold moreefficiently than the shorter 3 kB APP mRNA (Sauvage, et al., EMBO J11:3099 (1992). There are also two studies indirectly suggestingtranslational regulation of APP protein synthesis. Steady-state levelsof APP in the rat cerebral cortex, meninges, and in primary astroglial,microglial, and neuronal cultures do not reflect APP-mRNA levels(LeBlanc, et al., FEBS Letts. 292:171 (1991)). Furthermore, the relativelevels of APP-695 (KPI−) and APP-751 (KPI+) mRNA and their protein werediscordant in human brain. Each message is approximately equallyabundant whereas KPI+ proteins are the predominant (>82%) (Van Nostrand,et al., Proc. Natl. Acad. Sci. U.S.A. 88:10302 (1991)).

There are striking overlaps in the regulation of the ferritin and APPgenes. APP and ferritin are both acute phase reactants (APRs) regulatedat the translational level in hepatoma cells (Rogers, et al., J. Biol.Chem. 265:14572 (1990)) and primary astrocytes by IL-1. The APP mRNA 5′leader is organized into two regulatory sequences—an iron responsiveelement (IRE) at the 5′ cap site which is responsive to iron, oxidativestress (Pantopolous, et al., EMBO J. 14:2917 (1995)) and thyroid hormonereceptor (Leedman, et al., J. Biol. Chem. 271:12017 (1996)) and adownstream acute box sequence which is both a baseline and IL-1dependent translation regulatory element that works in an iron dependentfashion. The 5′ UTR of the APP gene has an effective acute box sequencein front of the start codon. However, the APP mRNA 5′ leader contains anoverlapping sequence upstream of the acute box which is related to theiron responsive element in ferritin mRNA (Klausner, et al., Proc. Natl.Acad. Sci. USA 93:8175 (1996)).

IL-1 enhancement of ferritin mRNA translation in hepatic cells and APPprotein synthesis in astrocytes suggests that the accumulation of Aβpeptides into plaques during Alzheimer's disease (AD) may be acceleratedby a pattern of local protein synthesis in glial cells, similar to ahepatic-style acute phase response. This model of elevated local APPprotein synthesis by a cytokine-mediated mechanism is consistent withincreasing experimental and epidemiological evidence linking Alzheimer'sdisease pathology to inflammatory mechanisms. Epidemiological studiesshow that non-steroidal anti-inflammatory drugs reduce the risk fordeveloping Alzheimer's disease (Andersen, et al., Neurology 45:51(1995)). The over-expression of interleukin-1 by centrally locatedmicroglia has been shown to be associated with early forms of amyloidplaques, the non-neuritic diffuse plaques, as well as being strikinglyincreased during plaque development (Das, et al., Neuron 14:447 (1995)).Thus, IL-1 has been suggested as a driving force for amyloid plaquematuration mediated by signaling of the cytokine to astrocytessurrounding the plaque structures and subsequent induction of APP andACT protein synthesis (Hentz, et al., Proc. Nat'l Acad. Sci. USA 93:8175(1996)). Recently IL-1 injection into the parenchyma of rat cerebralcortex was shown to increase the steady-state level of APP-protein atthe site of lesion (Sheng, et al., Neurobiol. Aging 17:761 (1996)). Theresults herein reinforce the view that IL-1 affects APP proteinsynthesis is increasing APP-mRNA translation.

All references cited are fully incorporated by reference. Having nowfully described the invention, it will be understood by those of skillin the art that the invention may be performed within a wide andequivalent range of conditions, parameters, and the like, withoutaffecting the spirit or scope of the invention or any embodimentthereof.

1. A method for assaying a test compound for its ability to alter theexpression of the human amyloid precursor protein, comprising: a)preparing a vector for recombinantly expressing a peptide or protein ina eukatyotic cell, wherein said vector comprises: i) a promoter which isactive in said eukaryotic cell; ii) a translation enhancer elementconsisting essentially of the nucleotide sequence of SEQ ID NO:1,wherein said element is 3′ to said promoter; iii) a DNA sequenceencoding said peptide or protein, wherein said DNA sequence: aa) lies 3′to said translation enhancer element; bb) is operably linked to saidpromoter; and cc) is non-homologous to said translation enhancerelement; b) measuring the expression of said peptide or protein in saidvector in the absence of said test compound; and c) comparing theexpression determined in step b) with the expression of said peptide orprotein in the presence of said test compound.
 2. The method of claim 1,further comprising transforming a host cell with said vector prior tomeasuring the expression of said peptide or protein.
 3. The method ofclaim 1, wherein said test compound comprises a nucleic acid sequencecomplementary to a region of SEQ ID NO:1 at least 10 base pairs inlength.
 4. The method of claim 1, wherein said test compound is an RNAtargeting compound.
 5. The method of claim 2, wherein said host cell isan astrocyte or astrocytoma cell.
 6. The method of any one of claims1-5, wherein said assay is carried out in the presence of one or morecytokines.
 7. The method of any one of claims 5, wherein said assay iscarried out in the presence of either interleukin-1α or interleukin-1β.8. The method of any one of claim 1-5, wherein said method includesmeasuring mRNA levels of said protein or peptide.
 9. The method of claim8, wherein said assay is carried out in the presence of one or morecytokines.
 10. The method of claim 8, wherein said assay is carried outin the presence of either interleukin-1α or interleukin-1β.